Heretofore, polyoxymethylene resin composition have been widely used in various mechanical working components and OA appliances as an engineering resin having balanced mechanical properties and wear resistance. In the production of polyoxymethylene resin, formaldehyde is usually used as a monomer, but due to wear in the molding machines, for example, in the extruder barrels, as a result of their use over a long period of time, thereby making the resin stagnant at some positions therein or due to some improper setting of temperature in the granulation or molding step or being left standing in a heated state for a long time, formaldehyde monomers will be sometimes released from the resin as a formaldehyde gas, resulting in a strong possibility of deteriorating the working circumstances in the production and molding of polyoxymethylene resin.
As a method of reducing the amount of released formaldehyde, JP-B-34-5440 discloses a method of adding a polyamide copolymer thereto; JP-B-39-21118 discloses a method of adding polysulfonamide thereto; JP-B-42-17107 disclose a method of adding a copolymer of acrylamide and styrene or vinylnaphthalene thereto; JP-B-43-14329 discloses a method of adding a copolymer with an acrylamide thereto; JP-B-43-22670 discloses a method of adding a superpolyamide thereto; JP-B-45-1752 discloses a method of adding polyamidothiophenol thereto; JP-A-51-64558 discloses a method of adding a polyamide copolymer having a melting point of not more than 180° C. thereto; JP-A-54-13551 discloses a method of adding a polyamide and a fatty acid thereto; JP-B-55-24465 discloses a method of adding an alkylurethane and a carboxylic acid amide thereto; JP-A-59-213752 and U.S. Pat. No. 5,015,707 disclose a method of adding a poly-β-alanine copolymer thereto; JP-B-2-11625 discloses a method of adding polytetramethylene adipamide particles pulverized to particle sizes of not more than 10 μm thereto; U.S. Pat. No. 5,106,888 discloses a method of adding a non-meltable polymer having aldehyde reacting groups thereto; etc. However, even addition of any of these materials to the polyoxymethylene resin has not been fully successful for preventing the release of formaldehyde gas from the resin.
On the other hand, JP-A-8-311302 discloses a method of adding at least one polymer selected from the group consisting of N-vinylcarboxylic acid amide homopolymers and copolymers of N-vinylcarboxylic acid amide and other vinyl monomers to the polyoxymethylene resin. Such a polymer can indeed more effectively absorb the formaldehyde gas than the aforementioned materials, but still suffers from such problems that discolored materials, which seem to be formed by heating, etc. of polymer per se in the extruder, molding machine, etc., (such material will be hereinafter referred to as “scorching”) are formed in the resulting composition or moldings, considerably deteriorating the appearance and resulting in inferior products. Furthermore, such a polymer has a high water absorbability per se, and thus there is a strong possibility that portions of the moldings may undergo swelling in contact with water during use in water or in high-humidity outdoor circumstances, or the like or there is a strong possibility of changes in mechanical and physical properties. Still furthermore, the recent trend of using moldings made from polyoxymethylene resin compositions is shifting toward thinner, more size-reduced mechanical working components, etc. and thus further improvement in physical properties against repeated stresses has been so far keenly desired.
In the production and molding of polyoxymethylene resin compositions there have been so far no technologies available for reducing the amount of released formaldehyde gas, suppressing “scorching” generation and improving the water resistance and repeated impact resistance of the resulting moldings.
The present invention has been accomplished to overcome the aforementioned problems and an object of the present invention is to provide a polyoxymethylene resin composition capable of reducing the amount of released formaldehyde gas and suppressing “scorching” generation during the production and molding of polyoxymethylene resin compositions and also improving the water resistance and repeated impact resistance of the resulting moldings.